Frequency modulation is one of the most common methods of transmitting analog voice signals across a communication channel. This method has been used in a wide variety of applications including cellular communications systems in the United States, Japan, and Europe.
A conventional analog approach to receiving FM signals includes a radio frequency (RF) front end that uses a heterodyne circuit to step down a received signal to an intermediate frequency (IF). In the receiver back end, an analog FM demodulator converts the signal to baseband using a discriminator with a tuning coil. Next, an analog processor in the receiver back end filters and adjusts the gain of the signal from the analog demodulator to produce a recovered signal that is then sent to an audio speaker. This analog approach, however, has several drawbacks due to the tuning requirements of the analog discriminator and the direct current (DC) offsets inherent in analog circuitry.
A digital approach to receiving FM signals, on the other hand, avoids the aforementioned problems of analog discriminator circuitry. In this situation, the RF front end is identical to an analog RF front end, while the receiver back end includes a digital FM demodulator that converts the IF signal to the digital domain, extracts the modulating signal, and sends it to a digital processor for filtering and gain adjustment. The output of the digital processor is then converted back into the analog domain, amplified, and sent to an audio speaker. Unfortunately, size and current drain requirements typically render conventional digital FM receivers impractical for low-cost, portable applications.
Thus, there is a need for an FM receiver that not only avoids the problems common to analog discriminator circuitry, but also offers a reduced size and power consumption alternative to conventional digital FM receivers.